Conventionally, there are portable terminal devices that execute control to make received voices easy to hear. For example, there is a technology with which multiple single-tone frequency signals are reproduced for a user to calculate the minimum hearing level based on the user's hearing result for processing a voice (Patent document 1).
Also, there is a technology in which a Lombard effect is utilized so that if a sending sound volume is loud, it is determined that it is noisy in the surroundings to increase the received sound volume, if a sending sound volume is soft, the received sound volume is decreased automatically (Patent document 2).
Also, there is a technology in which an equalizer is provided for emphasizing a voice signal in a specific range, and characteristics of the equalizer are adjusted based on a volume operation by a user (Patent document 3).